1. Field of the Invention
The present invention relates to an imaging apparatus using a solid-state imaging means having a photoelectron multiplying section, and an image obtaining apparatus using the imaging apparatus.
2. Description of the Related Art
An imaging apparatus for picking up an optical image of an observation area using a solid-state imaging device, such as a CCD and the like, that converts the optical image to an electrical signal has been known. Recently, a charge multiplying solid-state imaging device has been developed. This type of imaging device multiplies signal charges obtained by the device based on a multiplication factor which is controlled by a multiplication factor control signal as described, for example, in Japanese Unexamined Patent Publication No. 7(1995)-176721. Thus, the mounting of this type of imaging device on an imaging apparatus may enhance and control the imaging sensitivity of the apparatus. More specifically, the use of this type of solid-sate imaging device allows the user to obtain an image under the condition in which the amount of light is insufficient for a conventional imaging device, and display it as a visible image, as well as appropriately controlling the imaging sensitivity in accordance with the imaging conditions. This type of charge multiplying solid-state imaging device having the charge multiplying means described above is called a CMD (Charge Multiplying Detector)—CCD, in which signal charges are multiplied by making use of the charge multiplication effect arising from the ionization caused by the collision of conduction electrons with atoms under a high intensity electric field.
In the charge multiplying solid-state imaging device, signal charges are multiplied at a stage prior to a charge detection circuit that sequentially converts the signal charges to a signal voltage, and outputs it as an output signal, so that readout noise generated in the charge detection circuit is not multiplied, thereby the signal-to-noise ratio of the output signal may be improved. Accordingly, the use of the charge multiplying solid-state imaging device with an imaging apparatus which needs to pick up an optical image under insufficient light for the image may improve the signal-to-noise ratio of the output signal.
Also, endoscope systems having a solid-state imaging device mounted thereon have been used widely. These systems have an advantage that a number of individuals may simultaneously observe an image obtained by the solid-state imaging device by displaying it on the monitor. In addition, the image can be displayed on the monitor as a highlighted image by processing it in various ways in advance, thereby greatly contributing to the progress in the medical services.
In recent years, the endoscope has been applied in the field of bronchial tube, otolaryngology, and joint, as well as a conventional field of digestive system, with an evolution toward a thinner endoscope. The thinner endoscope, however, limits the number of light guides carrying the illumination light, so that there may be a case in which a sufficient amount of light may not be irradiated. Thus, the development of an imaging apparatus capable of picking up an image with a desired imaging sensitivity has been anticipated. In addition, a fluorometric observation, in which the excitation light described above is irradiated on a living tissue, and the fluorescence generated by the tissue is observed, has also been practiced. The fluorescence generated by the living tissue is weak, so that there may be a case in which a fluorescent image may not be picked up. Therefore, the development of an imaging apparatus capable of picking up an image with a desired imaging sensitivity has been anticipated. The US Patent publication No. 20010743994 discloses an endoscope system having a charge multiplying solid-state imaging device mounted thereon for solving these problems.
The use of the charge multiplying solid-state imaging device reduces readout noise and results in an output signal with an improved signal-to-noise ratio. Meanwhile, the noise contained in the output data obtained by the charge multiplying solid-state imaging means is dominated by dark noise. Further, if an image is picked up under a high temperature, by a solid-state imaging means having a large full well size, or under the condition in which a long read time is required for the signal charges, the dark noise contained in the signal charges is multiplied. For this reason, in the charge multiplying solid-state imaging device, the dark noise contained in the signal charges are multiplied with the multiplication of the signal charges, so that the signal-to-noise ratio of the output signal may not be improved even if the readout noise is reduced.
It may be possible to cool the solid-state imaging device for this purpose. It is difficult, however, to provide a cooling device for the solid-state imaging device mounted, for example, on an endoscope which is to be inserted into a living body. Further, the endoscope has a problem that the dark noise is inevitably increased due to a temperature rise caused by the illumination light irradiated on the subject at the time of imaging.